Because of their relative ease of fabrication on large area substrates thin film transistors (TFTs) have been actively studied for use in driving individual pixels in large area displays, such as liquid crystal displays. The TFTs generally comprise laterally spaced apart source and drain electrodes, held at different potentials, and electrically interconnected by a semiconductor material which forms a channel therebetween. Current flow between these electrodes, is controlled by the application of a voltage to a gate electrode, which is adjacent a portion of the semiconductor channel and is insulated therefrom. The gate field acts to invert or accumulate a portion of the semiconductor material, thereby conductively coupling the source and drain electrodes.
Amorphous silicon technology was initially developed primarily for photovoltaics but recently the microelectronic applications therefor have become more and more important. This material is ideally suited for use in large area arrays because of the low deposition temperatures involved in its fabrication and the availability of large area deposition and lithographic equipment. A shortcoming of amorphous silicon is its relatively low electron mobility which limits the operating speed of these transistors. The operating speed may be improved and the output current may be increased by reducing the channel length L, because the transit time of electrons across the channel is proportional to L.sup.2 and the output current is inversely proportional to the channel length (1/L). Typically, lithographically produced amorphous silicon TFTs have channel lengths of about 10 .mu.m. Of course, the channel length could possibly be decreased substantially by using critical lithographic techniques developed for VLSI fabrication, but this solution is very costly and is impractical over the large areas contemplated (e.g. one foot square and larger). In fact, it is likely that a one micron feature size cannot be accurately maintained over very large areas.
Another consideration to be borne in mind with respect to amorphous silicon devices is that these TFTs operate with a field effect mobility of around 1 cm.sup.2 /volt-sec despite the fact that the amorphous silicon electron band mobility is 10 to 20 cm.sup.2 /volt-sec. This is due to traps in the material, in the form of localized tail states, which allow only a small fraction (about 10 to 20%) of the charge induced into the channel of these TFTs to become mobile carriers. For a 10 .mu.m feature size and gate width-to-length ratio of 10, one may expect output currents on the order of 10 to 50 .mu.amp and a transit time of approximately 100 nsec for drive voltages (V.sub.DS) in the range of 10 to 20 volts. In practice, switching speeds also will be reduced by circuit capacitances. Improved current drive capabilities necessitate shorter channel lengths. Although amorphous silicon has been the most promising semiconductor material for TFTs, silicon in its polycrystalline or microcrystalline states may be fabricated over large areas and yield attractive results, and other semiconductor materials, such as Ge, GaAs and CdS, in these three states also have been found to be satisfactory.
A vertical, short channel thin film transistor, whose effective channel length is much less than any minimum lithographic feature size used in its fabrication, is disclosed in a copending application assigned to the same assignee as the instant invention. The copending application, entitled "High Current Thin Film Transistor" (Hack et al), filed on Mar. 29, 1988 and bearing Ser. No. 07/174,652 is fully incorporated herein by reference.
Thin film amorphous silicon sensors and phototransistors are well known. In large area applications, where transistors and sensors are fabricated upon the same substrate, such as, for example, in an optical scanning array, it would be convenient to configure these devices so as to have compatible fabrication techniques.
Therefore, it is an object of the present invention to provide a single vertical thin film structure which will operate as either a short channel, high current, thin film transistor or a high gain optical sensor in which barrier elements for improve the ON/OFF current ratio of both the transistor and sensor devices.
It is another object of this invention to provide a source electrode structure which introduces a lateral electric field in order to increase the transient response time of the sensor.